I won’t sidetrack this discussion onto another issue, but suffice it to say, I disagree and I consider the cleric, warlock and paladin examples to be rules.
But to answer your actual question, the consequence for a druid putting on metal armor would be on a par with a cleric blaspheming against their god, a paladin breaking their oath, or s warlock ignoring their patron.
But to turn the question back on you, my impression is that you agree that a cleric blaspheming, a warlock ignoring or a paladin violating their oath could have consequences (and that this doesn’t violate player agency). How are druids putting on metal armor any different from those examples?
Edit: Sorry, I realized that I mistook you for
@Charlaquin. Still interested in your perspective.